In high-voltage applications in power electronics, such as high-voltage d.c. transmission (HVDCT), static var compensation (SVC), and high-voltage variable-speed drives, a plurality of power semiconductors with their cooling systems, control electronics, and relieving networks are usually connected electrically in series to achieve adequate dielectric strength. The combination of power semiconductors and their associated cooling systems, control electronics, and relieving networks is known as a thyristor station. A plurality of such thyristor stations are combined mechanically in one module, and the design of the module must meet certain mechanical requirements and other requirements regarding the electric insulation of the components.
Such a module is described in the journal IEEE Transactions on Power Apparatus and Systems, vol. PAS-94, no. 3, May/June 1975, pages 1061-1071, and from the two embodiments of low and high direct voltage discussed in the Siemens brochure "High-voltage direct-current transmission (HVDCT)," order no. E50001-U131-A35, imprint PA 3942.
The article cited above concerns a high-voltage direct-current transmission system (HVDCT system) whose valves of one phase are arranged in an oil tank. These oil tanks are set up outdoors. Each valve comprises up to twenty power semiconductor modules. Also described are a relieving network, a block diagram of a control circuit, and the tank arrangement. The tank with its oil filling assumes the function of electrically insulating the thyristor stations with respect to the mechanical construction of the modules and the entire valve, and shielding all components from environmental influences.
In the conference contribution with the title "State-of-the-Art Thyristor Valves for China's First HVDC Transmission System" of the IEEE/CSEE Joint Conference on High Voltage Transmission Systems in China, Beijing, Oct. 17-22, 1987, pages 520-528, a high-voltage direct-current transmission system is proposed using water-cooled valves that are accommodated by suspension in a structure. This conference contribution describes the design of a modular unit. In addition, the control electronics of a power semiconductor and the valve-base electronics for optical transmission of control pulses are described.
The Siemens brochure "Advanced Series Compensation," order no. E50001-U132-A22-X-7600, imprint PA 1932, describes another embodiment of an arrangement of power semiconductor modules in which the modules switch capacitors and are arranged in a special walk-in container on an insulated platform together with the capacitors and reactance coils of a series reactive-power compensation system.
Certain technical problems stem from the electrical insulation of thyristor stations. These problems arise when several thyristor stations are connected electrically in series to achieve a high dielectric strength. Additional technical problems also arise with respect to measures intended to protect against environmental influences. In these measures, converters are arranged in framework structures and are accommodated with air insulation in a special structure, container or cabinet, or they are arranged in tanks filled with air or an insulating liquid. These embodiments have the disadvantage that relatively expensive structures are required even for low-power converters, and these often require complicated approval proceedings. Another problem is that the tank that is filled with insulating fluid or gas must be opened to replace individual defective components. Such replacement operations can be very labor-intensive.